Objective: To establish a practical, safe, reliable method for prolonged assistance to the failing circulation. Method: Our current system for providing heparinless extracorporeal circulation consists of a circuit consisting of a pump lined with dacron velour and employing pig valves used as inlet and outlet valves and plastic cannulae and tubing coated with nonthrombogenic polyurethane-polyvinyl-graphite material. This system will be used in our animal experiments for closed chest left heart bypass without anticoagulation. The experiments will be performed on three groups of sheep. Group 1 (20 Sheep). Effects of prolonged (7 days) closed-chest left heart bypass without anticoagulation on hemodynamics, blood cells, blood coagulation factors, and plasma proteins will be studied. Histological studies will be included. Group II (50 sheep). In order to standardize a technique to induce cardiogenic shock without thoracotomy, the coronary artery will be temporarily occluded by a balloon-tipped catheter directed under fluoroscopy. Ischemic time and site of occlusion in relation to the anatomical distribution of the coronary branches will be varied to result in 80% mortality. Group III (20 sheep). Animals subjected to experimental cardiogenic shock by the above technique will be treated with closed-chest left heart bypass without heparin. The hemodynamic effects and survival rate will be studied. Survivors will be kept for two months during which time treadmill and coronary angiography studies will be performed. The animals then will be sacrificed and pathological studies performed.